So I thought I would try and connect to Fan0, this worked in so far as if printing a filled square the Laser would switch ON at the square edge and then immediately OFF. This happens on both edges. Also the power seems to be low, it's not even marking a sheet of paper.
This is the config for the Fan0 setup:-
M555 P2 ; Set output to look like Marlin
G21 ; Work in millimetres
G90 ; Send absolute corrdinates...
M83 ; ...but relative extruder moves
M574 X1 Y2 S1 ; set endstop configuration (Y endstop only, at high end, active high)
M92 X87.4890 Y87.4890 Z4000 ; Set axis steps/mm
M92 E430 ; Set extruder steps per mm
M906 X950 Y950 Z950 ; Set motor currents (mA)
M201 X500 Y500 Z15 ; set accelerations
M203 X9000 Y9000 Z100 ; set maximum XYZ speeds
M566 X300 Y300 Z30 ; Maximum instantaneous speed changes mm/minute
M208 X200 Y200 Z190 ; set axis maxima
M208 X0 Y0 Z0 S1 ; set axis minimum (adjust to make X=0 the edge of the bed)

I have something very strange.
M3 S255 turns the laser OFF as does M3 S0
It appears that the maximum power is around M3 S127

Try lowering or increasing the PWM frequency with the M452F parameter. Some of the laser module controllers are a bit picky. You can also use the R parameter of M452 to set the PWM level that corresponds to max, so in your case, R127.

I tried changing the frequency but could not see any improvement.
I am currently using M3 S107 as my maximum power.
It works but I would prefer to have 255 levels of power, especially engraving photos.
Any other ideas as to how I can use the full range?

I've just measured the voltage on the Fan0 pin and it is fluctuating between 5.5V - 10.7V.
Do you think I am over driving the PWM input?
The problem I had no information supplied with the 2000mw laser and control board that I bought.
I have tried to find out the correct voltage but was unsuccessful.

I've just measured the voltage on the Fan0 pin and it is fluctuating between 5.5V - 10.7V.
Do you think I am over driving the PWM input?
The problem I had no information supplied with the 2000mw laser and control board that I bought.
I have tried to find out the correct voltage but was unsuccessful.

The ones I've seen have all been 5v PWM controlled, so maybe you're over-driving? Do you have a link to where you bought it? And/Or some high res images of the top+bottom of the PCB?

I bought it on ebay and the supplier has no information.
I could take some photos of the board but I will need to dismantle my setup first.
I wonder what size of resistor I could connect in series to reduce the actual voltage reaching the control board

I still haven't got the heater 3 solution as explained in the article "Controlling a PWM Laser with the Duet Board" The diagram indicates that the expansion header pin 8 is heater 3 but I have a constant 3.29 volts between that and pin 2 which is grd.
So I am wondering if the old version 6 boards, which I am using, have the same pin assignments as the version 2 boards.